Pharmaceuticals targeting signaling pathways of endometriosis as potential new medical treatment: A review

Sze Wan Hung, Ruizhe Zhang, and Chi Chiu Wang participated in research design. Sze Wan Hung participated in data evaluation, extraction and interpretation. Sze Wan Hung, Ruizhe Zhang, and Zhouyurong Tan participated in data validation and in drafting the manuscript. Tao Zhang participated in designing figure 1. Sze Wan Hung, Tao Zhang, Jacqueline Pui Wah Chung and Chi Chiu Wang critically revised the manuscript. All authors approved the final version of the manuscript.

In the treatment of EM, targeting a specific pathway, or multiple pathways alleviate the lesions. Targeting a single molecule can lead to several anti‐EM effects, as downstream transduction elements are usually connected to a series of molecular events as secondary responses. However, owing to synergistic effects, a multiple target therapy may have a greater suppressive effect on lesions compared with a single targeted therapy. 196 Table 4 summarizes several single pharmaceuticals with multiple molecular targets, which affect multiple signaling pathways in a complex disease such as EM. Pharmaceuticals that hold multiple molecular targets to different pathophysiology for endometriosis treatment Abbreviations: AKT, protein kinase B; AMPK, adenosine monophosphate‐activated protein kinase; CASP, caspases; CHOP, CCAAT/enhancer‐binding protein homologous 10 protein; COX, cyclooxygenase; DPPH, 2,2‐diphenyl‐1‐picrylhydrazyl; E 2 , Estrogen; EGCG, epigallocatechin gallate; ER, estrogen receptor; ERK, extracellular signal‐regulated kinase; ESR1, estrogen receptor 1; HMGB1, high mobility group box 1; H 2 O 2 , hydrogen peroxide; IKKB, IκB kinase beta; MAPK, mitogen‐activated protein kinase; MMP, matrix metallopeptidases; NAC, N‐acetyl cysteine; NF‐κB, nuclear factor κB; NK cells, natural killer cells; NOD2, nucleotide‐binding oligomerization domain‐containing protein 2; Nrf2, nuclear factor erythroid 2–related factor 2; O 2 , oxygen; OH, hydroxide; P450AROM, aromatase; PI3K, phosphoinositide 3‐kinases; PR, progesterone receptor; REDD1, protein regulated in development and DNA damage response 1; ROS, reactive oxidative stress; SIRT1, sirtuin 1; TCM, traditional Chinese medicine; TGF, transforming growth factors; TNF, tumor necrosis factor; VEGF, vascular endothelial growth factor; VEGFR, vascular endothelial growth factor receptor; Wnt, wingless‐type mouse mammary tumor virus integration site family. Melatonin is a natural substance produced by plants. It is also a hormone produced in the pineal gland to regulate neuroendocrine functions and inhibits LH and FSH secretion from the anterior pituitary gland. 293 Melatonin acts as an antioxidant and anti‐inflammatory agent and is currently under phase 2 clinical trial for reducing EM‐related pain. Another randomized, double‐blind, and placebo‐controlled clinical trial of melatonin was completed in 2013. The results of the study showed that melatonin acts as an analgesic and can relieve EM‐related chronic pain. 294 Melatonin receptor (MR)1A and MR1B are significantly upregulated in peritoneal EM lesions compared with those in the eutopic tissue. Melatonin has been shown to reduce EM lesions in various studies. It inhibits cell proliferation and modulates endometrial epithelial cell function. 295 Melatonin also inhibits angiogenesis via VEGF and oxidative stress via regulating radical scavenging activity and amplifies apoptotic activity via CASP3 mediated pathway in vivo and in vitro in EM. 63 , 142 , 143 Melatonin has no adverse effects on reproductive functions, instead, it can improve ovarian functions, and thus has potential to treat EM‐related infertility. 296 , 297 High‐dose intravenous treatment of pain and sepsis with melatonin showed no adverse effects. 298 Its bioavailability is 15%. 299 Long‐term therapeutic investigation of melatonin in EM should be conducted to elucidate its ability to regulate E 2 functions in EM. Metformin was shown to target multiple pathways 300 by regulating stromal‐epithelial cell communication in EM via Wnt2‐mediated signaling 48  and exerted an anti‐inflammatory effect through regulating cytokines. 149 Although a mild side effect was implied, 149 metformin regulated reproductive functions, 301 and improved conception in EM patients by inhibiting serum cytokine production. 149 Metformin is available in the market as a treatment for type 2 diabetes and PCOS in women. Considering its low cost, metformin was advocated to be used as a long‐term treatment. 302 NAC, an acetylated form of cysteine, has been prescribed as an antidote since the 1960s. It replenishes intracellular glutathione levels and modulates the redox environment; therefore, NAC is a strong antioxidant. 303 In EM, it acts as an antioxidant, antiproliferative, anti‐inflammatory, and anti‐invasiveness agent via ROS‐scavenging mechanism or through regulating cytokines in vitro and in vivo. 137 , 144 NAC is highly efficacious at low doses, and with no adverse effects in EM. 144 Long‐term adverse effects are also limited, including no effect on fertility. 137 , 144 NAC is considered to have a good safety profile and has been evaluated in phase 4 clinical trials for treating gastrointestinal and metabolic diseases. 303 , 304 Its pharmacokinetics and toxicity profiles are available; its terminal half‐life is 6.25 h after oral administration and bioavailability is 9.1%. 305 NAC is commercially available and cost‐effective as a dietary supplement in the market; however, studies on its efficiency in EM are limited, requiring more preclinical evidence. Natural products have a long history of use in the management of medical conditions. Research advances in analytical and synthetic chemistry have improved the identification and isolation of active compounds from natural products. EGCG is a polyphenol catechin from green tea and a well‐known antioxidant. It exerts efficacy against diseases including cancer, diabetes, and inflammation. 306 In EM, it exerts ant antiangiogenetic effect via the VEGFC/VEGFR2 pathways, 70 , 129 antioxidant effects via ROS‐scavenging mechanism, 130 antiproliferative effect via reduction of E 2 production, and anti‐migration and anti‐invasion effects via TGF‐β1‐induced phosphorylation of ERK1/2 and MAPK pathways, 167 thus inhibiting the development and growth of lesions. Promising evidence of its high potency and efficacy, and without major side effects in reproductive functions were reported. 130 , 167 EGCG is currently under phase 2 clinical trial for reducing lesion size and pain as well as an evaluation of its safety profile in EM. On the contrary, EGCG act as an adjuvant that brings synergistic effects, as well as reduces adverse effects in cancer treatment. 307 This suggests a potential role of EGCG in combination therapy with current EM treatment. However, the low bioavailability of EGCG has limited its attractiveness in the market. 308 ProEGCG, is a prodrug of EGCG, shows higher bioavailability and greater efficiency than EGCG to reduce lesions in vivo. 130 More studies should be conducted to confirm the underlying mechanism of ProEGCG in the treatment of EM. Resveratrol is a polyphenol found in grapes. In EM, it reduces proliferation via an anti‐E 2 mechanism targeting ESR1, 66 inhibits inflammatory responses via radical scavenging, 138 and inhibits angiogenesis by reducing the cytokines COX‐2 and VEGF 150 and activating SIRT1. 151 Resveratrol has completed a phase 4 clinical trial in EM and is safe and effective in relieving EM‐related pain, as well as in reducing serum CA125 and prolactin levels. Resveratrol is well‐known for its chemopreventive property. It also has a promising clinical profile in cancer treatment, nevertheless, the rapid metabolism rate of resveratrol has limited its efficacy in vivo. 309 Curcumin, genistein, ginsenoside, and puerarin are not under any EM clinical studies but have been clinically evaluated in breast cancers, endometrial carcinoma, endothelial functions, and so forth. They have been studied for their action mechanism against EM in primary cells, cell lines, and animal models. Curcumin, which is found in ginger and turmeric, enhances apoptosis by increasing the Bax/Bcl2 ratio through targeting of MMP‐3 via NF‐κB 81 and regulates angiogenesis and inflammation by targeting chemokines and cytokines 155 in EM. It has multiple biological effects in different diseases, including cancer and inflammatory diseases. It establishes a good safety profile with no acute toxicity. 310 Genistein is an isoflavone that acts as an E 2 agonist or antagonist to manage postmenopausal symptoms. In EM animal models, genistein downregulated MMP‐2/‐9 and regulated cell invasion and migration by targeting NF‐κB. 311 It also regulated NF‐κB by inhibiting TGF‐β. 85 Long‐term treatment with genistein lower the incidence of endometrial hyperplasia and provided support for bone formation in postmenopausal women. 83 , 312 It acts as a chemopreventive and chemotherapeutic agent against cancers and has synergistic effects with other anticancer drugs. 313 Toxicity of high dose is minimal, so it needs more study to test the safe range. 314 Ginsenoside RG3, extracted from ginseng, restored TNF‐α‐induced effects by inhibiting NF‐κB, VEGF, and CASP‐3 in EM, which are responsible for cell proliferation, apoptosis, and angiogenesis. 79 Ginsenoside PPD regulated ERα and PRα expression to suppress autophagy and lesion growth in EM. 103 It also targeted E 2 ‐induced NK cell cytotoxicity to regulate the immune system in EM. 103 Ginsenoside also possess synergistic effect with anticancer drugs, as well as prevents toxicity and morbidity from chemotherapy. 315 Puerarin is a phytoestrogen, binds to ERs via the ERK pathway to regulate proliferation in EM. 93 It also regulates inflammation in ectopic endometrium by inhibiting P450AROM and COX‐2 and promoting ERβ expression to facilitate E 2 metabolism in EM. 152 Its therapeutic effects are studied extensively in diseases including cancer and cardiovascular disease. 316 Most of these products have known toxicity or pharmacokinetic profiles and act via multiple targets, making them beneficial as anti‐EM agents. However, their poor aqueous solubility and low oral bioavailability in vivo is the major challenge to be potential EM treatment. 82 , 92 , 315 There are several approaches available currently to progress the bioavailability of drugs, which include prodrug approach, 130 solid dispersions approach, 317 lipid‐based formulation approach. 318 On the contrary, the long‐term safety of natural products in reproductive function and EM recurrence profiles should be further elaborated in future studies. Nevertheless, minimal side effects and available as an over‐the‐counter dietary supplement and routine remedies make them preferable to hormonal medicines. EM is a complex clinical challenge, and recently, more signaling pathways have been identified to contribute to its pathophysiology. EM drugs that target only one receptor have inadequate therapeutic efficiency. However, although multitarget drugs present potent efficacy in suppressing the progression of EM lesions, they also pose a risk of side effects such as binding to undesirable drug targets and bringing off‐target toxicities. 56 Therefore, designing a drug that targets the appropriate pathways with high selectivity is highly desirable. For this purpose, it is essential to understand the compound‐target pathway‐disease relationships.

This is the first review article combining medicinal research based on EM pathophysiology and the related signaling pathways. Our review revealed the challenges in EM management and the need for various available medical treatment options. Most of the medications prescribed by the FDA to treat EM are hormonal, such as contraceptives, progesterone, and GnRH. However, current hormonal medicines raise a major concern in the case of long‐term treatment. Therefore, new nonhormonal pharmaceuticals with relatively safer and few side effects are urgently needed. Our aims in this review were to facilitate the research and development of novel treatments for EM based on an understanding of the pathological process. To compare new and old pharmaceuticals, an effective scale to evaluate parameters between different treatments as well as to align outcome measures from preclinical to clinical studies is needed. There is a lack of experimental and clinical evidence to support the effectiveness, pharmacokinetic, and pharmacodynamic profiles of potential drugs in alleviating the pathophysiology of EM, compared with that of drugs already available in the market. Good practices such as the Endometriosis Phenome and Biobanking Harmonization Project, derived by the World Endometriosis Society, can help facilitate a large‐scale collaboration project worldwide. 337 It is a platform to ensure that the protocol is sufficient and consistent enough to maintain high research quality, datasets are shared to ensure data reproducibility, and results can better support the development of translational medicine. Moreover, multicenter collaboration can increase research visibility and avoid data integrity issues. The nonhormonal treatments reviewed in this paper were only studied in vitro or in animal models or are still under clinical trials. The drugs mentioned in this review article showed significant efficacy in reducing ectopic endometrium cell viability and endometriotic lesion size; however, severe adverse effects were not elaborated in‐depth. High efficacy and innovative approach do not guarantee final success. Data from legal regulation and patients’ demand for available resources are as important as the pharmacological profile of medicines. In many countries, a new drug must be regulated and approved by the relevant authority before it is launched in the market. 338 Thus, apart from efficacy and safety data, the medical and financial burden of EM to women have raised the awareness on EM and accelerated the scientific research on this disease, which are key factors considered by R&D investors. To maximize a drug's value and cost‐effectiveness in the market while maintaining its affordability, fulfilling the society's demand, and making scientific advances, modification of lead compound or bioactive compound derived from natural products holds great potential because only the functional groups are modified, whereas the original core structure is conserved. Considering both the medicinal and commercial perspectives of drug development, there is a huge pressure in the development of a new drug, starting from the synthesis or discovery stage to clinical trial, to proceeding with legal regulations, and to launch in the market. A drug requires 10–17 years of development, with less than 10% success rate to pass clinical trial. 339 Taking advantage of big data mining, drug repurposing is a strategy to identify new therapeutic use of a drug that is approved or under clinical trial, which comprises 30% of newly FDA‐approved drugs and vaccines. 340 , 341 These drugs can bind to the same target owing to the similar pathophysiology of different diseases, or these drugs can have multiple targets and are thus relevant to other diseases. 342 A repurposed drug offers sufficient preclinical pharmacology profile and safety reports, leading to a greater potential for phase III and IV clinical trials, which can reduce the time of drug development and cost of investment. Nevertheless, some advantages of de novo drug development outweigh the benefits of drug repurposing. A constant influx of chemicals via synthesis or extraction from natural products offers novel medical options for patients. A growing understanding of the pathophysiology of EM favors structure‐based or ligand‐based drug designs, in which by modifying lead compounds based on structure–activity relationships, the efficacy, potency, and selectivity can be compromised. However, a more in‐depth research is needed to study the underlying mechanisms and drug targets to support their potential as new EM treatments. In conclusion, this review provides an update on the pathophysiology of EM and shows the efficacy of various medicines in treating EM. Increasing attention has been focused on understanding the pathophysiology of EM and the action mechanisms of potential pharmaceuticals; however, many of these are still not completely understood. With this review, we hoped to raise awareness on the missing puzzle pieces and to promote related research that can further advance diagnosis and treatment for better management of EM and improve the quality of women's lives.

The choice of investigation models considerably influences the translational potential of preclinical research. Endometriotic and endometrial tissue cells with specific cell characteristics, defined by their morphology and phenotypes, confirmed by immunocytochemistry allow in vitro investigations of the mechanism of hormonal expression, cytokine secretion, cell proliferation, and differentiation. 42 Romano et al. 43 critically analyzed different EM culture models of samples from peritoneal, ovarian, and deep infiltration EM and recommended a guideline for assessing the quality of both primary endometriotic cells and immortalized endometriotic cell lines. Culture conditions can imitate EM in situ; for example, endometrium undergoing menstruation, 44 macrophage activation, 45 epithelium mesothelium transformation, 46 and cell–cell interactions. 47 , 48 In addition, in vivo animal experiments provide a biological system with an integrative environment and complete cellular and molecular network for lesion development and growth in vivo. It mimics the conditions in humans in the hopes that the results can be translated from bench to clinic. The application and limitations of various EM animal models, including autotransplantation of uterine tissues and xenotransplantation of human endometrial tissues into the peritoneal cavity or subcutaneous pocket in ectopic sites of rodent models, as well as in the primate model have been assessed, and the choice of the appropriate model for studies depends on the research questions. 49 Apart from the appropriate model, positive control of current pharmaceuticals should be included for comparison, which will serve as experimental evidence of the efficacy of new drugs. When choosing a positive control, pharmaceuticals with relevant actions to the examined molecular and signaling pathways should be considered. For example, dienogest can be used as a positive control to compare the inhibition of NF‐κB activation, enhancement of apoptosis, or inhibition of MMP‐2/‐9, 50 , 51  leuprolide acetate to compare the inhibition of promitogenic cytokines, 52  and celecoxib to compare the proliferation‐inhibitory and apoptosis‐enhancing effects. 53 In addition to the efficacy, the pharmacokinetic profile of a drug with respect to absorption, distribution, metabolism, and excretion should be available to support its clinical use. 54 The bioavailability of a drug and its active metabolites in systematic circulation and local tissues should be quantified to justify the therapeutic dosage for clinical application. 55 The relationship between drug potency and pharmacological effects on the body and action site should be evaluated to prevent off‐target toxicities. 56 The possible adverse effects on other tissues also need to be determined. Medications with specific efficacy on the ectopic endometrium and minimal side effects on the eutopic endometrium are preferable for EM treatment, as these medications will affect reproductive cycles the least. In animal experiments, adverse effects on reproductive tissues and functions should be carefully monitored. As a short‐term measure, no significant change in body weight should occur in the test animals, and as a long‐term measure, the animals should be able to conceive and deliver. For women with EM who prefer symptomatic medical therapy, such side effects should be limited and well‐tolerated. Medicines that regulate E 2 levels usually result in hypoestrogenism and are associated with side effects such as hot flushes and vaginal dryness, which are acceptable, but not preferable. 57 Other common adverse effects, such as osteoporosis and venous thromboembolism, should be avoided. The effect of the medications on fertility should also be monitored; however, the current data are very limited. Several studies have systematically recorded the direct and indirect costs of EM treatment and highlighted its long‐term economic burden on the society, healthcare system, and affected women. 58 This has raised awareness of the disease and increased the demand for cost‐effective EM drugs. However, the choice of treatment depends not only on patients’ desired outcomes but also on treatment affordability. A cost‐effective medication is that with equivalent monetary value and efficiency. Therefore, as the most cost‐effective treatment for EM is considered for use as a standard hormonal treatment, 59 a potential new pharmaceutical should be affordable and easily accessible to the market, in addition to showing good efficacy with fewer side effects. In summary, a potential new pharmaceutical should be well‐studied in terms of not only action mechanism and efficacy in vitro and in vivo, but also safety, efficiency, and cost‐effectiveness. Progress in this area is expected to provide clear and effective insights for policy‐making and for decision‐making in the individualized treatment of EM.

In the theory of Chinese medicine, EM is defined as a blood stasis syndrome that leads to the formation of endometriotic lesion and other associated symptoms. Stagnation of Qi (energy) is believed to be one of the causes of EM. TCM aims to lessen the chronic pain experienced by women with EM. Therefore, studies on the action mechanism of TCM are focused mainly on the alleviation of inflammation and oxidation. TCM decoctions containing several herbs in different compositions, which are varied according to the condition of the patient, are a combinational approach that can target various pathophysiology. Fang et al. 319 and Tsai et al. 320 have identified the decoctions commonly used for treating EM in Taiwan, which included Gui‐Zhi‐Fu‐Ling‐Wan, Dang‐Gui‐Shao‐Yao‐San, Jia‐Wei‐Xiao‐Yao‐San, Shao‐Fu‐Zhu‐Yu‐Tang, and Wen‐Jing‐Tan. The therapeutic efficacy and pathophysiology of TCM in cancer and other diseases have been widely evaluated in vitro and in vivo; however, there are limited studies on the efficacy of TCM for EM. Most of the herbs exert anti‐inflammatory effects by inhibiting the production of proinflammatory cytokines. Poria has been confirmed to exert antitumor activities against various cancers. It binds to cytokines and effector immune cells to regulate immunity and upregulate apoptosis. 321 Angelicae Sinensis Radix exerts anti‐inflammatory effects by reducing TNF‐α inflammatory cells. 322 Ligusticum Rhizoma inhibits inflammation and reduces PGE 2 production. 323 Moutan Cortex, Glycyrrhizae Radix, Paeoniae Alba Radix, and Bupleuri Radix suppress proinflammatory cytokines via the NF‐κB signaling pathways. 324 , 325 , 326 , 327 Paeoniae Alba Radix and Bupleuri Radix also exert such effect via MAPK signaling pathways. Atractylodis Ovatae Rhizoma exerts antioxidant effect by activating the MAPK cascades and inhibiting the production of radicals by 2,2‐diphenyl‐1‐picrylhydrazyl and catalases, thus inhibiting the activity of free radicals. 328 Glycyrrhizae Radix and Poria act as radical scavengers against superoxide and hydroxy radicals. 328 , 329 , 330 Ligusticum Rhizoma acts as a reducing agent via the Nrf2 and NF‐κB pathways. 331 Angelicae Sinensis Radix exerts antiproliferative and proapoptotic effects; it induces mitochondrial‐dependent apoptosis and inhibits the Akt/mTOR pathway. 332 Atractylodis Ovatae Rhizoma induces apoptosis by upregulating ROS. 333 Moutan Cortex exerts proapoptotic effects by increasing Bax/Bcl‐2 expression and decreasing MMP via the formation of apoptosome and cytochrome c , activation of CASP, and the adenosine monophosphate‐activated protein kinase pathway. 323 It also induces apoptosis via activation of CASP‐3/‐8. 334 Paeoniae Alba Radix induces apoptosis via activation of CASP‐3/‐9 293 and exerts antiproliferative activity via cell cycle arrest and Fas/Fas ligand‐mediated apoptotic pathway. 334 It also downregulates the antiapoptotic protein Bcl and upregulates the apoptotic proteins Bax and CASP‐3. 335 TCMs have great potential as multitarget drugs. As TCMs consist of herbal formulas with various combinations of herbs, they have multiple mechanisms of action, which can be beneficial to reduce the concentration of each herb, thus, drug toxicity. 336 However, the costs and availability vary for different herbs, which limits its acceptability in Western countries at present. Furthermore, there is a lack of clinical management methods to evaluate their clinical effectiveness and standardized regulations of TCM practice.

EM is a disease caused by functional endometrial tissues growing in other areas outside the uterine cavity. It is a chronic disease that affects productivity and quality of life in women. 1 The typical presenting symptoms in women with EM include chronic pelvic pain, abnormal menstruation, and dyspareunia. EM occurs frequently in women of reproductive age, and the incidence is approximately 10%. 2 Approximately 40%–60% of women with EM experience dysmenorrhea, and 20%–30% are complicated with infertility. 3 Although EM presents as benign clinical and pathological manifestations, it has similar characteristics to cancers, including dissemination, invasion, and hyperplasia. It is generally accepted that EM is a hormone‐dependent disease. 4 Estrogen (E 2 ) augmentation and progesterone resistance feature EM pathology, but the mechanism of how this occurs is unclear. Nevertheless, EM has been observed even in the absence of increased E 2 production in postmenopausal women. 5 The pathogenesis of EM is dominated by the theory of ectopic implantation of the endometrium, along with multiple factors, such as endocrine, immunity, invasion, and angiogenesis. Retrograde menstruation theory suggests reflux of endometrial tissue through the fallopian tubes during menstruation and implantation into the peritoneal cavity. 6 , 7 Lymphatic and vascular dissemination theories suggest that endometrial cells disseminate via lymphatic or blood circulation. 8 Stem cell origin theory suggests that undifferentiated peritoneal tissue, ovarian surface epithelial tissue, and endometrium mesenchymal stem cells transform into endometrial‐like tissue in response to retrograde menstrual blood flow and stimulation from chronic inflammatory factors. 9 EM development is also associated with a combination of genetic variation and environmental factors. First‐degree relatives of women with EM have a seven fold greater risk of developing EM than those without a family history, and the risk of developing the disease in identical twins of women with EM is as high as 75%. 10 , 11 In recent years, the increased incidence of EM is also thought to be associated with exposure to environmental pollutants. Tetrachlorodibenzo‐p‐dioxin (TCDD) is the most prevalent air pollutant worldwide, and it promotes cytokine secretion. Endogenous E 2 exacerbates the effects of TCDD and the interaction of the two chemicals provokes inflammatory responses, induces toxicity, and thus increases the severity of EM. 12 , 13 , 14 Therefore, the pathophysiology of EM is complex, interrelated, and specific, thereby requiring multiple targeted therapies. Regardless of EM theories, endometrial cells must complete a serial process of immune escape, survival, adhesion, invasion, and angiogenesis to develop and grow in the ectopic sites. 15 Signaling pathway refers to a series of enzymatic reaction pathways that pass molecular signals into cells through the cell membrane to exert corresponding effects. EM‐related signaling pathways, together with their upstream and downstream regulatory factors, constitute a large and complex transduction system and play an important role in the occurrence and development of EM. Abnormalities in these pathways and their interactions can lead to abnormal proliferation, apoptosis, autophagy, adhesion, invasion, fibrosis, angiogenesis, reactive oxidative stress (ROS), immune system, and inflammatory responses of the ectopic endometrial tissues, thereby promoting its growth and development. Hormonal‐related enzymes, growth factors, inflammatory cytokines and chemokines, such as tumor necrosis factor (TNF)‐α, transforming growth factors (TGF)‐β, prostaglandin E 2 (PGE 2 ), prostaglandin‐endoperoxide synthase (COX)2 play important roles in these processes. 16 , 17 They induce local immune imbalance in the microenvironment to tolerate immune clearance and promote the survival of ectopic lesions. Downstream molecules, such as hypoxia‐inducible factors (HIF)‐1α, matrix metallopeptidase (MMPs), and vascular endothelial growth factors (VEGFs), are dysregulated and play roles in the angiogenesis and growth of EM lesions. 2 , 15 , 16 , 17 Current treatments for EM include surgical and medical therapies. Conservative surgery removes the EM deposits but increases the risk of impairing ovarian reserve, harming other organs, and imposing postoperative recurrence. 18 Therefore, medical therapy (Table 1 ) always comes first into consideration, and the choices depend on multiple factors, such as symptom severity, conceive desire, and comorbidities. Generic classes of medical therapies for EM include hormonal therapy, including oral contraceptives (COC), progesterone and gonadotropin‐releasing hormone (GnRH) agonist and antagonist, and nonhormonal therapies such as nonsteroidal anti‐inflammatory drugs (NSAIDs). Current FDA‐approved medication for endometriosis treatment Tolerable side effects. Cost‐effective. Combined use of progestin with ethinyl estradiol reduces adverse effects such as thromboembolism. Tolerable side effects. Cost‐effective. Combined use of progestin with ethinyl estradiol reduces adverse effects such as thromboembolism. Side effects related to hypoestrogenism, such as hot flashes, dry vagina, nausea, headaches, and so forth. Adverse effect associated with long‐term usage such as thromboembolism and stroke. High recurrence rate after discontinuation. Risk of impaired fertility Side effects related to hypoestrogenism, such as hot flashes, dry vagina, nausea, headaches, and so forth. Adverse effect associated with long‐term usage such as thromboembolism and stroke. High recurrence rate after discontinuation. Risk of impaired fertility Available in different forms of administration and in different price ranges. Intramuscular injection form of treatments avoids daily administration and reduces gastrointestinal absorption. High specificity and minimal side effects with Dienogest. Available in different forms of administration and in different price ranges. Intramuscular injection form of treatments avoids daily administration and reduces gastrointestinal absorption. High specificity and minimal side effects with Dienogest. Side effects related to hypoestrogenism, such as hot flashes, dry vagina, nausea, headaches, and so forth. Adverse effect associated with long‐term usages such as reduction in bone mineral density and virginal bleeding. Side effects related to hypoestrogenism, such as hot flashes, dry vagina, nausea, headaches, and so forth. Adverse effect associated with long‐term usages such as reduction in bone mineral density and virginal bleeding. Available in different forms of administration and in different price ranges. Direct effect on endometriotic tissues. Approved add‐back therapy can reduce side effects. Available in different forms of administration and in different price ranges. Direct effect on endometriotic tissues. Approved add‐back therapy can reduce side effects. Side effects related to hypoestrogenism, such as hot flashes, dry vagina, nausea, headaches, and so forth. Aromatase inhibitors need to be taken to prevent initial pituitary flare effect. Side effects related to hypoestrogenism, such as hot flashes, dry vagina, nausea, headaches, and so forth. Aromatase inhibitors need to be taken to prevent initial pituitary flare effect. Long history—First approved drug for EM. Long history—First approved drug for EM. Side effects related to hyperandrogenism such as hirsutism and muscle cramps. Increased risk of ovarian cancer. Replaced by alternative agents due to adverse effects. Side effects related to hyperandrogenism such as hirsutism and muscle cramps. Increased risk of ovarian cancer. Replaced by alternative agents due to adverse effects. Lower degree of hypoestrogenism side effects compared to GnRH agonists. Flexible and rapid reversible onset and offset. Lower degree of hypoestrogenism side effects compared to GnRH agonists. Flexible and rapid reversible onset and offset. Side effects related to hypoestrogenism, such as hot flashes, dry vagina, nausea, headaches, and so forth. Adverse effect associated with long‐term usage such as reduction in bone mineral density. Side effects related to hypoestrogenism, such as hot flashes, dry vagina, nausea, headaches, and so forth. Adverse effect associated with long‐term usage such as reduction in bone mineral density. Convenient as one injection per month. Convenient as one injection per month. Side effects related to hypoestrogenism, such as hot flashes, dry vagina, nausea, headaches, and so forth. Adverse effect associated with long‐term usage such as osteoporosis. Expensive Side effects related to hypoestrogenism, such as hot flashes, dry vagina, nausea, headaches, and so forth. Adverse effect associated with long‐term usage such as osteoporosis. Expensive Combined use of Leuprolide with Norethindroe prevents bone thinning. Convenient as one injection per month. Combined use of Leuprolide with Norethindroe prevents bone thinning. Convenient as one injection per month. Side effects related to hypoestrogenism, such as hot flashes, dry vagina, nausea, headaches, and so forth. Side effects related to hypoestrogenism, such as hot flashes, dry vagina, nausea, headaches, and so forth. Abbreviations: COC, combined oral contraceptive; E 2 , estradiol; FDA, Food and Drug Administration; FSH, follicle‐stimulating hormone; GnRH, gonadotropin‐releasing hormone; LH, luteinizing hormone; LHRH, luteinizing hormone‐releasing hormone; NETA, norethindrone acetate; NSAID, nonsteroidal anti‐inflammatory drug; P4, progesterone; P450AROM, aromatase. Data was extracted from The Drugs.com Database, drugs.com . Price range was justified based on 3‐months therapy. $ denotes the approximate price range and are labelled as follows, $ ($5000). Medication is usually prescribed together with NSAIDs. The available reports on the effectiveness of NSAIDs on pain relief in EM are very limited, and there is no strong evidence to support a conclusion. 1 Among all medical treatments, combined COC and progestin monotherapy represent the first‐line therapy, which can be applied to most women clinically diagnosed with EM with or without a surgical diagnosis. 26 Continuous COC effectively reduces the recurrence of dysmenorrhea, 27 and progestin suppresses ovulation by maintaining a hypoestrogenic state. Women with risk factors such as thrombosis and myocardial infarction may tolerate the side effects of progestin better than those of COC. 28 To date, few derivatives of progesterone, namely, depot medroxyprogesterone acetate and norethindrone acetate, have been approved by the US Food and Drug Administration (FDA) as the sole therapy for EM. 29 , 30 Although GnRH is an effective hormonal treatment for EM, severe hypoestrogenic symptoms limit long‐term compliance. 31 , 32 GnRH agonists are second‐line hormonal therapies that exert strong action on the GnRH receptor, leading to an initial short stimulation and subsequent suppression of gonadotropin secretion. Decreased hormone levels result in the dormancy of endometriotic lesions. Owing to its long‐term adverse effects, especially osteoporosis, an add‐back therapy is recommended. 33 Recently, the FDA approved elagolix, a nonpeptide small molecule GnRH receptor antagonist that suppresses luteinizing hormone and follicle‐stimulating hormone and correspondingly reduces E 2 and progesterone, as a treatment for moderate to severe EM‐associated pain. Its efficacy was shown after a 6‐month treatment, but it also caused a significant decrease in bone mineral density as the main side effect. 34 To overcome EM refractory to current hormonal treatments and NSAIDs, there have been extensive research of new medicines in recent years. Other than therapeutic efficacy, the potential use of a drug as a preventive treatment after surgery is also desirable. The recurrence of EM and the associated symptoms within 5 years after laparoscopy is approximately 19% in patients with endometrioma, 35 and up to 10% of women require secondary surgery after 1 year, 36 emphasizing the need for new medical treatments to prevent a recurrence. In summary, to identify and develop new pharmaceuticals for EM treatment, understanding the dysregulated molecular and signaling pathways in EM development is essential (Figure 1 ). Numerous studies have focused on the antiproliferation mechanism and related targeted therapies in EM models and/or endometrial cells. 37 , 38 Owing to the interaction of different signaling pathways, the efficacy of potential pharmaceuticals in promoting or inhibiting a single signaling pathway is often very limited. Therefore, pharmaceutical targeting multisignaling pathways in EM has become important in the medical treatment of EM. An overview of the molecular pathways involved in the pathophysiology of EM has been reported by various publications, 39 , 40 , 41 which provides a high quality evidence of the underlying pathophysiology of EM. However, previous publications only focused on currently available pharmaceuticals. In this review, we aimed to present an updated summary of studies focusing on new potential pharmaceuticals, including preclinical studies, clinical trials, as well as studies on marketed pharmaceuticals. In‐depth studies of signaling pathways targeted by pharmaceuticals are currently an emerging research direction, which will open up broad prospects for the new generation of EM treatment. Pathophysiology of endometriosis. The schematic diagram was created using BioRender.com . Akt, protein kinase B; ATG, autophagy‐related genes; DC, dendritic cells; E 2 , estrogen; ECM, extracellular matrix; ER, estrogen eceptor; ERK extracellular signal‐regulated kinase; FGF, fibroblast growth factor; FGFR, fibroblast growth factor receptors; HIF, hypoxia‐inducible factors; MΦ, macrophages; MAPK, mitogen‐activated protein kinase; MEK, ERK kinase; mTOR, mammalian target of rapamycin; NF‐κB, nuclear factor κB; NK, natural killer; PDGF, platelet‐derived growth factor; PDGFR, platelet‐derived growth factor receptor; PI3K, phosphoinositide 3‐kinases; Rho, Ras homolog family; ROCK, Rho‐associated coiled‐coil kinase; VEGF, vascular endothelial growth factor; TGF, transforming growth factor; TNF, tumor necrosis factor; Treg, regulatory T cells; Wnt, wingless‐type mouse mammary tumor virus integration site family; YAP, Yes‐associated protein

Chi Chiu Wang is an active member of the World Endometriosis Society and an advisor of the Aptorum Group.